MicroRNA (miRNA) fragments are non-coding RNA segments that appear to play an important role in translation regulation of messenger RNA (mRNA), and may be a contributing factor in various diseases including neurogenerative diseases. There are many different types of miRNA, and a given type of miRNA may play a regulatory role in multiple processes. The small size of miRNA (e.g., typically 8-22 base pairs) can make labeling and detecting these molecules difficult, because the miRNA molecules are often smaller even than typical probe molecules that are used for labeling.
A closely related family of molecules are termed small interfering RNA (siRNA). These molecules have comparable composition, size, and behavior to miRNA, but are not naturally occurring in the cell and are introduced by methods such as gene therapy.